
public class Person
{
  public int Id { get; set;}
  public string Name { get; set;}
  public int Age { get; set; }

}

// public implicit extension PersonExtension for Person
// {
//     public int Age => DateTime.Now.Year - this.Birthday.Year;
//     public bool IsStudent => this.Age < 18;
// }

// public explicit extension Student for Person
// {
//     public string Grade => "五年级";
// }